1. Field of the Invention
The present invention relates to the field of crystal oscillators and specifically to a crystal oscillator frequency stabilizing apparatus that utilizes a hybrid Temperature Compensated Crystal Oscillator (TCXO) and Oven Controlled Crystal Oscillator (OCXO) design in order to maintain the temperature of the oscillator within a given predetermined operating range as well as avoiding temperatures at which activity dips frequently occur.
2. Description of Related Art
Crystal-controlled oscillators comprise a large portion of the frequency sources that are used in many RF signal-transmitting devices including emergency personnel locating devices. In emergency personnel location devices, it becomes critical to incorporate an oscillator frequency source that provides a predictable stable oscillation frequency through varying ambient temperatures.
Aircraft emergency locator systems such as ELTs (Emergency Locator Transmitters) and terrestrial location systems such as PLBs (Personal Locator Beacons) each require stable frequency sources. Individuals lost at sea and fortunate enough to have emergency location identification devices such as an Emergency Position Indicating Radio Beacon (EPIRB), can send out RF signals with the hopes that their distress signals will be picked up by maritime stations or satellites, which can in turn, relay the information to the proper authorities.
Distress signals are transmitted at very specific frequency ranges. For example, 406 MHz is a specific frequency that is only allowed for emergency broadcast for aviation and maritime radio communication channels such as xe2x80x9cmaydayxe2x80x9d or xe2x80x9cSOSxe2x80x9d-related distress signals. Stable frequency sources are needed to support accurate beacon location. Satellites can locate the beacons by a Doppler measuring technique that gives the satellite a line of position when the satellite passes the beacon location. A major limitation on the accuracy of such Doppler locations is the frequency stability of the beacon transmitter. As an example, 406 MHZ signals which use the OCXO typically can be located to a position accuracy of 1 or 2 kilometers while signals at 121.5 MHz (another emergency broadcast frequency) which usually use unheated crystal oscillators can be located to a position accuracy of between 10 and 30 kilometers.
Many RF stable frequency sources incorporate electrical circuits that provide automatic frequency adjustment to compensate for temperature-driven frequency variations that commonly occur in quartz crystals. These frequency sources allow the crystal temperature to vary and then attempt to provide frequency correction by various tuning schemes, known in the art as Temperature Compensated Crystal Oscillators (TCXOs) and other closely related schemes such as Microprocessor Controlled Crystal Oscillators (MCXOs) and Digitally Compensated Crystal Oscillators (DCXOs).
Each of the above mentioned schemes share a common difficulty known as quartz crystal activity dips, which cause phase and frequency instability in the oscillator output, and in some severe cases, the circuit may cease oscillation entirely. These activity dips occur at discrete temperatures, which are usually unique to each crystal unit. The activity dips result from undesired modes of vibration within the crystal combined with the desired mode to either reinforce or interfere with desired mode resonance. The unique temperature behavior of each unit results from the desired mode and undesired modes having different temperature/frequency coefficients, and when the temperature is just right, the desired and undesired modes can have the same frequency and therefore interfere, causing large frequency resonance variations within the crystal.
Oven Controlled Crystal Oscillators (OCXOs) are stable frequency sources that incorporate oscillator circuits and crystal in a precisely temperature-controlled oven enclosure.
OCXOs are used where the ultimate in frequency stability is required. The degree of frequency stability achieved by an OCXO depends on the quality of the temperature control circuits, the design of the oven enclosure, arid most importantly, the match between the oven operating temperature and the zero temperature/frequency slope (turn-over) temperature of the crystal. However, accurate frequency control by OCXOs requires high power, large ovens, and very accurately processed and characterized crystals.
The present invention utilizes a TCXO in combination with an oven circuit thereby eliminating the need for exact control of the crystal cut to achieve a specific turn-over temperature and eliminating the requirement for precise setting and control of oven temperature matching the crystal turn-over temperature thereby allowing use of an inexpensive oven circuit and package.
Certain applications for frequency sources could be readily serviced by Temperature Compensated Crystal Oscillators (TCXOs) if the activity dip phenomenon could be eliminated. Unfortunately, there is presently no compensated scheme that is completely free of activity dips and the associated phase and frequency instability.
An additional problem with TCXOs is that TCXOs assume the local environmental temperature, typically between 40xc2x0 C. to 50 or 60xc2x0 C. At the temperature extreme, the rate of crystal frequency change with temperature change becomes excessive and impossible to accurately compensate.
The present invention incorporates an oven circuit that keeps the TCXO temperature away form activity dips that occur randomly over the oscillator temperature range thereby providing greatly improved frequency and phase stability compared to standard TCXO designs.
Additionally, although there have been attempts in the prior art to provide a hybrid TCXO-OCXO design where the heater element is used to keep the TCXO from going below a minimum temperature where it no longer functions, such as U.S. Pat. No. 6,060,692 issued to Bartley et al., such attempts fail to address the activity dip problem since the oven does not operate while the temperature is above the TCXO low temperature xe2x80x9ccriticalxe2x80x9d limit, and there is therefore no way to avoid activity dip temperatures that occur within the TCXO""s normal operating temperature range.
Further, prior art OCXO components are mounted on standard, thick PCB or ceramic substrates and substrate mounted to the case by heavy metal leads to support the substrate structure. The heavy leads used for mechanical support conduct heat away from the substrate causing oven current demand to increase substantially.
Accordingly, what is needed in the art is a crystal oscillator frequency stabilizing apparatus that incorporates the beneficial features of TCXO and OCXO circuits in order to maintain the temperature of the oscillator within a given predetermined operating range as well as avoiding temperatures at which activity dips frequently occur and a method of constructing an oscillator circuit wherein the design of the substrate results in minimal heat loss through the substrate and surrounding components.
The present invention is a crystal oscillator frequency stabilizing apparatus and method of constructing same that combines an inexpensive TCXO design with a miniature, inexpensive, oven circuit design whereby the TCXO is kept in a small high temperature range just above the operating temperature range and away from activity dips of the TCXO crystal. The TCXO compensates for allowed temperature variation resulting from ambient temperature changes and small, low power, inexpensive ovens. The overall result is a frequency source that offers greater stability than the TCXO without the instability of activity dips. Finally, the present invention provides a method for constructing oscillators with a heating circuit providing minimal heat loss through the substrate material.
An improved stabilized frequency source comprising a substrate, preferably having a thickness between 0.008 and 0.015 inches, a temperature compensated crystal oscillator (TCXO) disposed upon a first surface of the substrate, a heating control circuit for maintaining the temperature of a TCXO within a predetermined temperature range and outside of activity dip temperature ranges, where the heating circuit is disposed upon the substrate""s second surface, substrate supporting means for supporting the substrate, the temperature compensated oscillation circuit and the heating element within an enclosure, and an enclosure housing the substrate and the circuits disposed thereon wherein the substrate, temperature compensated crystal oscillator circuit and heating circuit are not in contact with any portion of the enclosure.
Unlike the prior art, which uses a piezoelectric crystal with precisely controlled turn-over temperature in conjunction with a precision oven, the present invention uses a TCXO replacing the piezo crystal, within a simple oven circuit. By using a TCXO, the need for tight tolerance crystal turn-over temperature control and tight tolerance oven temperature control is eliminated, by using an oven circuit to keep the TCXO within a small temperature range just above the maximum system operating temperature, frequency instability due to activity dips is eliminated while the cost is much less than a conventional OCXO.
In the preferred embodiment, the supporting means comprises a plurality of support pins protruding through corresponding apertures within the base of the enclosure and the substrate whereby the substrate, temperature compensated crystal oscillator circuit and heating circuit are suspended within the enclosure without making contact with any portion of the enclosure.
The temperature compensated crystal oscillator circuit is comprised of a piezoelectric crystal, an oscillator circuit and a thermistor temperature compensation network electrically coupled thereto. The heating circuit is comprised of a temperature sensor electrically coupled to a temperature control amplifier, and a heating element, such that upon certain ambient temperature variations, said temperature control amplifier receives a signal from the sensor and varies the heating element accordingly to maintain the substrate at approximately constant temperature.
Preferably, the temperature compensated crystal oscillator circuit and the heating circuit are each soldered on opposing surfaces of the thin substrate thereby providing a thermal but not an electrical connection between the circuits.
The enclosure is a standard resistance welded metal enclosure hermetically sealing the components within.
The present invention also provides a method of stabilizing the frequency of a crystal oscillator, comprising the steps of providing a substrate having a first surface and a second surface, disposing a crystal oscillator circuit upon the first substrate surface, disposing a heating circuit for maintaining the temperature of a crystal oscillator within a predetermined temperature range and outside of activity dip temperature ranges, upon the second substrate surface, providing substrate supporting means for supporting the substrate and circuits thereon within an enclosure, and housing the substrate and the circuits disposed thereon within an enclosure wherein the substrate, crystal oscillator circuit and heating circuit are not in contact with any portion of the enclosure.
The present invention is an apparatus, which incorporates an inexpensive temperature compensated oscillation circuit, with a miniature, inexpensive, low precision oven circuit. Each circuit is disposed upon opposite surfaces of a very thin insulated substrate, and the substrate suspended away from the interior of the enclosure that houses the apparatus. The design avoids activity dips that often occur at discrete temperatures.
It is an additional feature of the present invention to provide a simple design that allows for the construction of electronic component heating assemblies that can be made by using conventional PCB techniques and automated SMT assembly while providing the desired high thermal coupling between the TCXO and oven circuits while providing low heat loss through the PCB material to the mounting structure.
In the preferred embodiment, the circuit means of the radio beacon includes a frequency source stabilizing apparatus comprising: a substrate having a first surface and a second surface; a temperature compensated crystal oscillator circuit disposed upon the first substrate surface; a heating circuit for maintaining the temperature of a crystal oscillator within a predetermined temperature range and outside of activity dip temperature ranges, said heating circuit disposed upon the second substrate surface; substrate supporting means for supporting the substrate, the temperature compensated oscillation circuit and the heating element; and an enclosure housing the substrate and the circuits disposed on the substrate wherein the substrate, the temperature compensated crystal oscillator circuit and the heating circuit are not in contact with any portion of the enclosure.
It is an object of this invention to provide an apparatus and method that stabilizes the RF frequency of a piezoelectric quartz resonator in a simple and cost-effective manner.
It is another object of this invention to provide a method of constructing an electronic component heater assembly that provides a stable RF frequency source.
It is yet another object of the present invention to provide an emergency positioning radio beacon comprised of a frequency stabilizer apparatus that stabilizes the RF frequency of a piezoelectric quartz resonator in a simple and cost-effective manner.
It is to be understood that both the foregoing general description and the following detailed description are explanatory and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute part of the specification, illustrate the preferred embodiment of the present invention and together with the general description, serve to explain principles of the present invention.